Metal nanoparticles constitute an intermediate state between bulk materials and atomic or molecular structures. In contrast to bulk metals, which display constant physical properties regardless of their size, the properties of nanoparticles are highly dependent on their size. This size dependency is attributed to the nature of the nanoparticles, having a discrete and quantisized energy spectrum. Obeying quantum-mechanical rules, their electronic structures differ from those of bulk metal on one hand, and of molecular compounds on the other.
The characteristics of nanoparticles stem from various features such as particle size, shape and inter-particle distance. A particularly important feature is their surface to bulk ratio since the percentage of atoms at the surface increases significantly as the size of the material approaches the nanoscale. Furthermore, molecules that are attached to the surface play a key role in determining the physical and chemical properties of these particles. They are particularly important for their stabilization and are often referred to as organic coating.
The application of organic coating on nanoparticles has several functionalities. It modifies the chemical characteristics of “bare” nanoparticles and, most importantly, affects their electronic properties. Furthermore, it is also possible to obtain electron transport between the nanoparticles and the organic coating thus introducing cooperative effects. Nanoparticles capped with an organic coating (NPCOC) are therefore of high technological importance, particularly since there is a wide variety of compounds from which both the nanoparticles and their organic coating may be selected. Furthermore, the organic coating can be tailor-made in order to control the chemical and physical properties of the nanoparticles, in a qualitative as well as quantitative manner, to enhance desired properties of the NPCOCs including solubility, stability, etc.
The use of NPCOCs for sensing applications has many advantages. The sensing signal from an NPCOC sensing device can be easily obtained either by controlled aggregation (self-assembly) or by swelling of the NPCOC mainly through hydrogen-bonding. Other interactions, including π-π, van-der-Waals, electrostatic, charge-transfer, host-guest or antigen-antibody, may also contribute to sensing. Additionally, parameters which include, for instance, nanoparticles and/or aggregate size, inter-particle distance, composition, periodicity, and aggregate thermal stability can be manipulated in order to enhance the sensing signal. Enhanced selectivity can further be achieved through modifying the binding characteristics of the capping film as well as linker molecules. The morphology and thickness of NPCOC networks were shown to induce a dramatic influence on sensor response, wherein changes in permittivity induced a decrease in resistance of NPCOC thinner films (Joseph et al., J. Phys. Chem. C, 112, 12507, 2008).
Another advantage for the use of NPCOCs for sensing applications is increased sensitivity. This is mainly attributed to the three dimensional assembly of structures which provide a framework for signal amplifications. Other advantages stem from the coupling of nano-structures to solid-state substrates, thus providing easy array integration, rapid responses, and low power-driven portable format.
Chemical sensors based on NPCOCs can be fabricated as electronic nose devices. Such devices perform odor detection through the use of an array of cross-reactive sensors in conjunction with pattern recognition methods. In contrast to the “lock-and-key” model, each sensor in the electronic nose device is widely responsive to a variety of odorants. In this architecture, each analyte produces a distinct fingerprint from an array of broadly cross-reactive sensors. This configuration may be used to considerably widen the variety of compounds to which a given matrix is sensitive, to increase the degree of component identification and, in specific cases, to perform an analysis of individual components in complex multi-component (bio) chemical media. Pattern recognition algorithms can then be applied to the entire set of signals, obtained simultaneously from all the sensors in the array, in order to glean information on the identity, properties and concentration of the vapor exposed to the sensor array.
Sensors based on changes in the physical and/or electrical properties of films composed of spherical NPCOC (“SNPCOC”) were applied as chemiresistors, quartz crystal microbalance, electrochemical sensors and the like. Some examples for the use of SNPCOCs for sensing applications are disclosed in U.S. Pat. Nos. 5,571,401, 5,698,089, 6,010,616, 6,537,498; Patent Application Nos. WO 99/27357, WO 00/00808, FR 2,783,051 US 2007/0114138; and in Wohltjen et al. (Anal. Chem., 70, 2856, 1998), and Evans et al. (J. Mater. Chem., 8, 183, 2000).
U.S. Pat. No. 6,773,926 discloses sensors and sensor systems for detecting analytes in fluids, the sensors include a plurality of particles having one or more capping ligands coupled to a metallic core. Exposure of the sensors to a fluid containing a chemical analyte causes the analyte to react with the metal core, preferably by displacing one or more of the capping ligands. The chemical analyte can be detected through a change in electrical or optical properties of the sensors.
U.S. Pat. No. 6,746,960 is directed to techniques for detecting and identifying analytes in fluids. The system used therein comprises an insulating layer covering a conductive layer on a substrate. A sensor well comprising a sensor material whose electrical properties are changed in the presence of an analyte is patterned and etched in the insulating and conductive layers to enable analyte detection.
U.S. Pat. No. 7,052,854 discloses systems and methods for ex vivo diagnostic analysis using nanostructure-based assemblies comprising a nanoparticle, a means for detecting a target analyte/biomarker, and a surrogate marker. The sensor technology is based on the detection of the surrogate marker which indicates the presence of the target analyte/biomarker in a sample of a bodily fluid.
EP 1,215,485 discloses chemical sensors comprising a nanoparticle film formed on a substrate, the nanoparticle film comprising a nanoparticle network interlinked through linker molecules having at least two linker units. The linker units are capable of binding to the surface of the nanoparticles and at least one selectivity-enhancing unit having a binding site for reversibly binding an analyte molecule. A change of a physical property of the nanoparticle film is detected through a detection means.
Theoretical as well as experimental observations indicate that SNPCOC based sensors are typically limited to detecting analytes in a concentration range of 100-1000 parts per billion (ppb). This limitation has been attributed to two main reasons. First, while voids between adjacent (spherical) nanoparticles can host analyte molecules during the exposure process, they do not contribute to the obtained sensing signal. Second, the contact interface between adjacent spherical nanoparticles, onto which analyte molecules adsorb and induce sensing signals (e.g., by inducing swelling/aggregation of the film), is limited to a very small area in comparison to the total surface area of the SNPCOCs.
For the reasons mentioned hereinabove, obtaining high sensing performance requires an increase in film thickness. However, such an increase results in intensified diffusion limitations, thus reducing the response time. Hence, there is an unmet need for fast responsive sensors having improved sensitivity as well as selectivity. Furthermore, there is an unmet need for a reliable sensing apparatus to analyze either volatile or non-volatile compounds.